On 15/05/2021 11:27, Theo wrote:
John Rumm wrote:
It was quite common to have a big cast iron lump HX with high water
content (couple of gallons). That would have 4 bosses on it - two
outputs and two returns. One set would be used for the feed and return
on the pumped circuit through the rads, and the other two would be used
for a gravity (i.e. convected circuit) to heat an indirect cylinder.

Ah, so 'gravity' meant there was no pump on the HW side, only for the
heating?


Yes indeed, and no motorised valves either.


The convention circuit required that the hot water cylinder would be
placed some distance above the boiler and not too far displaced from it.
It would also typically be piped in 28mm pipe to better allow convection
flow. Hot water from the gravity output would rise up through the pipe,
and through the coil in the cylinder, the heavier cooler water would
fall and return to the boiler. Basically it sets up a thermosyphon.

That's neat. Horribly inefficient, but neat

Inefficient, yes because there was no boiler interlock. In other words,
the boiler continued to fire just to keep itself warm even when there
was no heating demand and the hot water was hot enough.

That sounds about right - cylinder was above the boiler and about 2.5m
displaced, so that would probably work.

Some refinements of the system introduced and anti gravity valve to stop
heat being lost out of the cylinder through the boiler when it was off.
Some had a kind of thermostatic valve in the gravity loop to limit the
maximum temperature of the cylinder DHW. Some would add a motorised
valve to make it a fully controlled zone. (aka C plan)

I never investigated but I imagine there was just on/off control for the
boiler (gas valve only, since there was an always-on pilot that had a piezo
spark-button to relight it if it went out), and an additional time clock
output that was in series with a thermostat to run the pump for the heating.

How was the boiler turned off in this system, when the thermostat said the
house was hot enough and so was the water? If the heating called for heat
and the hot water was up to temp, surely that would overheat the water?


The boiler had an electrically operated gas valve, so would only run
when power was applied - plus a safety interlock which prevented the gas
valve from opening unless the pilot was alight and heating its thermocouple.

As others have said, the hot water would eventually get up to boiler
temperature - which was too hot for comfort unless the boiler stat was
turned down. Then the radiators may not get hot enough. One solution -
which I installed in my first house in about 1969 - used a thermostatic
valve (a bit like a TRV) in the gravity return from the cylinder coil
and stopped the circulation when the hot water was hot enough. It didn't
stop the boiler cycling on its own stat, of course.

Such systems would typcially have a two-channel programmer
(electro-mechanical rather than digital of course!) to switch the boiler
on when hot water and/or heating were required and to switch the pump on
(often via a room thermostat) when heating was required.
--
Cheers,
Roger